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Milky Way sidelined in galactic tug of war
by Nicky Guttridge
Posted: 01 October 2010

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Gas formations and streams between and around the Magellanic Clouds originally attributed to the Milky Way’s gravity are now proposed to be due to interactions between the Clouds themselves.

The Large and Small Magellanic Clouds are two dwarf galaxies relatively close to the Milky Way. They display gas structures and formations spanning the distance between them and trailing behind them through space. The Magellanic Stream is an arc of hydrogen gas following behind the Milky Way’s neighbour galaxies. It spans over 100 degrees of sky, and the Milky Way’s gravitational pull was thought to be the dominating force in its creation through pulling gas from the two Clouds. Now, however, a new study suggests that it formed with no input from the Milky Way and is purely due to interactions between the Clouds themselves.

Click here for larger version. This plot shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) as they orbit our home Milky Way Galaxy. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. The colour range from dark to light shows the density (lower to higher) of the hydrogen gas making up the Magellanic Stream and the Bridge that connects the two dwarf galaxies. Image: Plot by G. Besla, Milky Way background image by Axel Mellinger.

The Stream’s formation was thought to be a result of the Milky Way’s gravitational influence as the Clouds completed an orbit around our Galaxy. Previous studies have shown that for this to be the case this orbit would have taken less than two billion years, but a research team led by Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics has ruled out this type of orbit. They instead suggest that the Clouds are not long-time satellites of the Milky Way and are in fact new arrivals either on their first passage, or on an eccentric long-period orbit of length greater than six billion years.

Without a short period orbit around the Milky Way, the stripping of the Magellanic Stream from the Clouds can no longer be attributed to our Galaxy’s gravity. A new computer simulation showed this was the case by assuming the Clouds to be a stable binary system on their first passage past the Milky Way. This simulation was backed up by measurements from the Hubble Space Telescope taken by Besla’s colleague, Nitya Kallivaylil. The team’s results indicated that the formation of these Magellanic gas structures had no dependence on the Milky Way, and formed before the Clouds were captured by our Galaxy.

“While the Clouds didn’t actually collide,” says Besla, “they came close enough that the Large Cloud pulled large amounts of hydrogen gas away from the Small Cloud. This tidal interaction gave rise to the bridge we see between the Clouds, as well as the Stream.”

There is evidence to suggest that the Magellanic Clouds have interacted in the past. They are connected by a bridge of gas, referred to as the Magellanic Bridge, which indicates that they are likely to be a binary interacting pair. Although the Milky Way may not have been the cause for the Stream and Bridge formations, its gravity now has a huge effect on shaping the orbit of the Clouds and so is responsible for the present-day appearance of the Stream. This can be shown through the line-of-sight velocities and spatial location of the Stream as it is observed today.

“We believe our model illustrates that dwarf-dwarf galaxy tidal interactions are a powerful mechanism to change the shape of dwarf galaxies without the need for repeated interactions with a massive host galaxy like the Milky Way,” adds Besla.

The paper fully detailing this work will be published in the October 1 issue of the Astrophysical Journal Letters.